Remote extension not registering

hii all,

i am using elastix server 2.3 and all my remote extensions were working good till last week. But for one particular country i have three extensions and now i am unable to register these remote extensions using yealink ip phone connected directly from modem .Using zoiper softphone i can register in mobile through wifi in the same remote network .How can i solve this issue

By providing sufficient diagnostics and information on what has changed that might indicate the cause.